This invention relates to an optical unit having an optical element such as a diffractive optical element, for example, and, more particularly, to an optical unit suitably usable in manufacture of a microdevice such as an IC or LSI, for example.
Semiconductor device manufacturing technology has recently been advanced remarkably and, in this trend, microprocessing techniques have been developed significantly. For microprocessing, reduction projection exposure apparatuses having a resolving power of submicron order and being called steppers are mainly used. For further improvement of resolution, enlargement of the numerical aperture (NA) of an optical system, shortening of exposure wavelength or introduction of a novel optical element has been attempted.
It has been proposed that a plurality of optical elements, each being such as a diffraction element (diffractive optical element) to be used for correction of chromatic aberration, for example, are used in an exposure apparatus.
Where diffractive optical elements are to be bonded with each other, the optical performance may be deteriorated unless they are relatively positioned with good precision.
It is an object of the present invention to provide an optical unit having desired optical performance, wherein diffractive optical elements are relatively positioned with good precision and are bonded with each other.
In accordance with an aspect of the present invention, there is provided an optical unit, comprising: a first optical element; and a second optical element; wherein said first optical element has a protrusion while said second optical element has a recess; and wherein the relative alignment between said first and second optical elements is accomplished by engagement of the protrusion and the recess.
Each of the first and second optical elements may comprise a diffractive element for diffracting a light ray incident thereon, at a predetermined deflection angle.
The diffractive element may comprise a diffraction grating (binary grating) having a step-like shape along an optical axis direction or a diffraction grating of Kinoform shape.
In accordance with another aspect of the present invention, there is provided an optical unit, comprising: a first diffractive element; and a second diffractive element; wherein said first diffractive element has a protrusion while said second diffractive element has a recess; wherein the relative alignment between said first and second diffractive elements is accomplished by engagement of the protrusion and the recess; and wherein each of said first and second diffractive elements comprises a diffraction grating having a step-like shape along an optical axis direction or a diffraction grating of blazed shape.
In one preferred form of these aspects of the present invention, the protrusion and the recess are formed at diffraction grating surfaces of said first and second diffractive elements.
The protrusion and the recess may be formed outside diffraction grating surfaces of said first and second diffractive elements.
The protrusion and the recess may be formed at centers of said first and second diffractive elements, respectively.
The first and second diffractive elements may have a plurality of protrusions and a plurality of recesses formed at positions corresponding to the protrusions.
The first and second optical elements may be bonded with each other with a predetermined spacing maintained therebetween, wherein, with respect to the spacing, the difference between an optical path length of a light ray passing through the protrusion and an optical path length of a light ray passing through a portion other than the protrusion may correspond to a multiple, by an integral number, of the wavelength of the light ray.
The first and second diffractive elements may be made of different mediums, wherein a space may be defined between a free end of the protrusion and a bottom of the recess, and wherein an optical path length as defined by a structure of the space, the protrusion and the recess may correspond to a multiple, by an integral number, of the wavelength of a light ray passing therethrough.
In accordance with a further aspect of the present invention, there is provided a method of manufacturing an optical unit, comprising the steps of: selectively removing a predetermined region on a first substrate to produce a step-like shape on the surface of the first substrate, while forming at least one recess upon the surface thereof; selectively removing a predetermined region on a second substrate to produce a step-like shape on the surface of the second substrate, while forming a protrusion upon the surface thereof; and engaging the recess and the protrusion so as to relatively position the first and second substrates, and adhering the first and second substrates with each other.
In one preferred form of this aspect of the present invention, the first and second substrates are made of different mediums, wherein the height of the protrusion is smaller than the depth of the recess.
In accordance with a further aspect of the present invention, there is provided an optical system which comprises an optical unit as recited above, and a lens.
In accordance with a yet further aspect of the present invention, there is provided an exposure apparatus having an optical system as above, for projecting and printing a mask pattern onto a surface to be exposed.
In accordance with a still further aspect of the present invention, there is provided a device manufacturing method, comprising the steps of: exposing a workpiece having a photosensitive material applied thereto, with a device pattern and by use of an exposure apparatus as recited above; and developing the exposed workpiece.